Recently, in applications such as label package doubling as a protection of a glass bottle and a PET bottle etc. and display of articles, cap sealing and accumulation package, there have been widely used, as a shrink label, a polyester-based heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in solvent resistance. The use amount of the polyester-based heat-shrinkable film tends to increase being accompanied by an increase in volume of PET containers.
Heretofore, a heat-shrinkable polyester film has been widely utilized which shrinks greatly in the width direction. It is also known that the shrinkage in the longitudinal direction, which is a non-shrinking direction, is made to be below zero (so-called extend due to heating) in order to achieve satisfactory shrinkage finishing properties (Patent Document 1). Although the heat-shrinkable polyester film in which the width direction is the main shrinking direction is subjected to drawing at a high ratio in the width direction in order to exhibit the shrinkage properties in the width direction, with regard to the longitudinal direction orthogonal to the main shrinking direction, there have been many cases in which the film is only subjected to drawing at a low ratio and there is also a case in which the film is not subjected to drawing. The film subjected to drawing at a low ratio in the longitudinal direction and the film subjected to drawing only in the width direction have a drawback that the mechanical strength in the longitudinal direction is poor. Moreover, when the film is subjected to drawing in the longitudinal direction in order to improve the mechanical strength in the longitudinal direction, the mechanical strength in the longitudinal direction increases, but the shrinkage in the longitudinal direction also increases, which in turn deteriorates shrinkage finishing properties.
Conventional heat-shrinkable films have been produced by adjusting the composition of polyester and the drawing conditions so that the hot-water heat shrinkage at 90° C. is 40 to 60% (Patent Document 2). With regard to the heat-shrinkable films having even higher shrinkage, the hot-water heat shrinkage at 90° C. is 40 to 80% (Patent Document 3), and a heat-shrinkable film having a hot-water heat shrinkage at 90° C. exceeding 80% has not been produced.
In recent years, for the purpose of protection of contents or improvement in design, there is a need to cover a major portion of a container with a label. Then, high shrinkable films having a shrinkage in the width direction exceeding 80% has been desired. On the other hand, when the shrinkage in the longitudinal direction is high, the length of the label in the longitudinal direction becomes short, which is contrary to the need to cover a major portion of a container with a label. Therefore, a need to make the shrinkage in the longitudinal direction become 0 or below zero (extend) has increased. However, with regard to the films with high mechanical strength in the longitudinal direction as in Patent Documents 2 and 3, the shrinkage in the longitudinal direction is not below zero.
It is contradictory that while the mechanical strength in the longitudinal direction maintains high, the shrinkage in the longitudinal direction is reduced below zero, and this is difficult. Further, when the shrinkage in the width direction is made higher, the shrinkage in the longitudinal direction also becomes higher, so that the film becomes inferior in shrinkage finishing properties.